1. Field of the Invention
The present invention relates to the filtering of fluids, and more particularly to the filtering of well fluids. Still more particularly, the present invention relates to the method of manufacturing of such filters.
2. Background and Related Art
Well filters are typically used in subterranean environments in which it is desired to remove a liquid or gas from the ground without bringing soil particulates, such as sand or clay, up with the liquid or gas. A well filter generally includes an inner support member, such as a perforated core, and a filter body including a filter medium disposed around the inner support member. In many cases, the well filter will further include an outer protective member, such as a perforated cage or shroud, disposed around the filter body for protecting it from abrasion and impacts. A filter for subterranean use is described in U.S. Pat. No. 6,382,318, hereby incorporated herein by reference. A downhole screen and method of manufacture is described in U.S. Pat. No. 5,305,468, hereby incorporated herein by reference.
In order to facilitate installation of the cage over the filter body, there is usually a clearance between the outer periphery of the filter body and the inner periphery of the outer protective member. This clearance between the filter body and the outer protective member or shroud is problematic. During use of the well filter in a well, fluid will usually flow radially inwards through the filter body. However, on occasions, such as during acidizing of a well, injection of mud into a well, air sparging, or momentary pressure reversals, fluid may flow radially outwards through the filter body, producing harmful hoop stresses which stretch the filter medium in the filter body outwards toward the outer shroud. Because of the clearance between the filter body and the cage, there is a likelihood of the hoop stresses exceeding the strength of some or all portions of the filter body and producing excessive plastic elongation of the filter medium or damage to seams or joints of the filter body.
It is known in the art to form the outer shroud separately from the filter body and slide it over an end of the inner support member until it surrounds the filter body. After the shroud is disposed around the filter body, the shroud is plastically deformed radially inwardly to reduce the inner diameter of the shroud to a value such that the filter body can expand against the shroud. This process is also know as “swaging,” and is typically achieved using a reducing mill or die. Swaging has several drawbacks, making it an undesirable step which should be avoided in manufacturing well filters. First, it adds to the time and expense of manufacturing the filter. Second, it deforms the outer shroud, putting additional stresses on the shroud and affecting its material makeup. Third, swaging does not guarantee that the clearance between the filter body and the shroud has been properly eliminated, or that the pressure on the filter body is desirable, especially considering the “spring-back” phenomenon. Spring-back occurs when the outer shroud expands slightly after the radial inward force exerted by the mill or die has been released.
Other common practices in the well filter industry also cause problems in manufacturing well filters. Often times the filter body, typically a mesh screen, is hand wrapped around the inner support. However, it is not possible to hand wrap the filter body as tight as is necessary, thereby creating unwanted slack in the filter. To combat this problem, the filter body is wrapped around the inner support longitudinally and a lengthwise weld is applied at the seam. Typically, a plasma or laser weld is used at the seam so as not to bum the fine mesh. However, this is time-consuming and expensive. Also, regardless of the type of weld used and care taken, welding causes the mesh in and around the weld to draw together, thereby causing the filter to bend and contort in undesirable ways. The weld also weakens the area around the weld and, over time, can cause erosion and breakage.
In prior art filters and methods, the filter body is usually bound to the inner support using a variety of techniques, including chemical glues and heat. In a process called sintering, the filter body is heated after it is in contact with the inner support. However, this technique does not bind the filter body to the inner support properly, and additionally plugs some of the small holes in the filter mesh and shrinks the filter body.
The industry would welcome a filter and method of manufacturing such a filter that eliminates the problems found with conventional filters and methods. The present invention overcomes the deficiencies of the prior art.